If you think there's something romantic about fireflies glowing on a warm summer night... You'd be right.

But what you don't see, is the dark side of this luminous display.

Firefly flashes are a secret code, a language of light.

The light comes from a masterful bit of chemistry.

A bioluminescent reaction that generates light but no heat.

So what are they saying?

Well, males on the wing are advertising themselves to females with a bit of sexy skywriting.

Take the common Eastern firefly.

His signature move?

A fishhook-shaped maneuver.

Which is why his species is sometimes called the "Big Dipper."

Her reply is more subtle: a single, slow pulse from her heart-shaped lantern.

Our "Big Dipper" comes bearing a "nuptial gift," a present of more than 200 assorted nutrients... kind of like a box of chocolates.

Here's the handoff.

Some are lucibufagins -- defensive chemicals fireflies secrete to ward off predators like spiders and birds.

These defensive chemicals may help protect her.

Firefly codes are so reliable that anyone can speak the language.

But we're not the only codebreakers listening in.

Meet Photuris.

She's also a firefly -- a larger, stronger one than the Big Dippers.

But she has a weakness.

Her species can't make its own lucibufagins.

They have fewer defenses against predators.

So she sets a trap to get some.

She mimics the glow of other firefly females - - luring in the males of that species.

When Mr. Big Dipper shows up with his chemical gift, she moves in... sucks up those defensive chemicals that she desperately needs... ...then makes a meal of the rest of him.

Most fireflies don't even eat during the few weeks they spend as adults.

But he's not totally defenseless.

If she's not quick enough, he can secrete a gooey compound that sticks in her jaw and lets him escape.

Another gift from the master chemist.